Optical elements are increasingly based on new materials possessing specific, precisely controllable properties. In particular, a necessary element in modern visual display systems is an optically anisotropic film with a combination of optical and other characteristics that may be optimized to suit the requirements of a particular device.
Various polymeric materials have been used in the manufacture of optically anisotropic films. Films based on such materials may acquire anisotropic optical properties through uniaxial extension and modification with organic dyes or iodine. In many applications, the base polymer is polyvinyl alcohol (PVA). Such films are described in greater detail in the monograph Liquid Crystals: Applications and Uses, B. Bahadur (ed.), World Scientific, Singapore—N.Y. (1990), Vol. 1, p. 101. However, the low thermal stability of PVA-based films typically limits their application. Development of new materials and methods for the synthesis of optically anisotropic films possessing improved characteristics is therefore quite advantageous. In particular, higher heat resistance, more convenient synthesis, and better film-forming properties are highly desirable.
Organic dichroic dyes have gained prominence in the manufacture of optically anisotropic films with improved optical and working characteristics. Films based on these compounds may be obtained through application of a layer of a liquid crystal (LC) aqueous dye solution containing supramolecules composed of dye molecules onto a substrate surface followed by evaporation of the solvent (e.g. water). The resulting LC films acquire anisotropic properties either through preliminary mechanical ordering of the underlying substrate surface as described, for example, in U.S. Pat. No. 2,553,961 or through subsequent application of external mechanical, electromagnetic or other orienting forces to the LC coating on the substrate as described, for example, in PCT Publication No. WO 94/28073.
Investigations into the application of LC dyes as described above, as well as the properties of related systems have become more extensive in the past decade. Recent studies into these phenomena have been motivated largely by industrial applications in liquid crystal displays (LCDs) and glazing. Dye supramolecules may form lyotropic liquid crystal (LLC) phases. In such phases, dye molecules pack into supramolecular complexes that are shaped generally like columns, which are the basic structural units of a mesophase. High ordering of dye molecules in the columns allows such mesophases to be used for obtaining oriented films characterized by a strong dichroism.
Dye molecules that form supramolecular LC mesophases typically include peripheral groups that render the dyes water-soluble. The mesophases of organic dyes are characterized by specific structures, phase diagrams, optical properties, and dissolving capabilities, as described for example in J. Lydon, Chromonics, in: Handbook of Liquid Crystals (Wiley—VCH, Weinheim, 1998), Vol. 2B, pp. 981 to 1007.
Using dichroic dyes capable of forming LLC systems, it is possible to obtain films possessing a high degree of optical anisotropy. Such films exhibit the properties of E-type polarizers, which are related to peculiarities of the optical absorption of supramolecular complexes, and behave as retarders (phase-shifting devices) in the spectral regions where the absorption is insignificant. The phase-retarding properties of these anisotropic films are related to their birefringence (double refraction), that is, a difference in refractive indices measured in the direction of application of the LLC solution onto a substrate and in the perpendicular direction. Films formed from LLC systems based on strong (light-fast) dye molecules are characterized by high thermal stability and light resistance.
The above properties of LLC systems account for the growing interest in these materials. Methods have been developed for preparing films based on such organic dyes. Recent improvements have involved both optimization of the film application conditions and identification of new LLC systems. In particular, new LLC compositions for the synthesis of optically anisotropic films may be obtained by introducing modifiers, stabilizers, surfactants, and other additives to known dyes so as to improve characteristics of the films as described in, for example, published PCT Publication No. WO 94/28073.
In recent years, there has been increasing demand for the films possessing high optical anisotropy that are also characterized by improved selectivity in various wavelength ranges. Films whose absorption maxima occur at different locations in the wide spectral range from the infrared (IR) to the ultraviolet (UV) are very desirable. A broad assortment of compounds capable of forming LLC phases and films possessing the required properties have been developed. However, the number of dyes known to form stable lyotropic mesophases is still relatively small.
Disulfoderivatives of various organic dyes, including perylenetetracarboxylic acid (PTCA) dibenzimidazole (DBI) are important water-soluble dichroic dyes capable of forming stable LLC phases. PCTA DBI species applicable in the manufacturing of optically anisotropic films have been described previously in PCT Publication No. WO 94/28073. In general, PTCA dibenzimidazoles and diimides are characterized by excellent chemical, thermal, and photochemical stability. These properties have triggered increased interest in these substances as potential materials for obtaining optically anisotropic films for LCDs and other optical devices.
Widespread use of these dyes has been hindered by their generally poor solubility in water as well as in some organic solvents. To provide for the solubility of perylene dyes in organic solvents, the introduction of various substituents into the initial molecules has been suggested. Examples of such substituents include oxyethyl groups as described in R. A. Cormier and B. A. Gregg, Phys. Chem. 101 (51), 11004 to 11006 (1997) and phenoxy groups as described in H. Quante H. Y. Geerts, and K. Mullen, Chem. Mater. 6(2), 495 to 500 (1997). Improved solubility of perylene dyes may also be provided by amino groups as described in I. K. Iverson, S. M. Casey, W. Seo, and S.-W. Tam-Chang, Langmuir 18(9), 3510 to 5316 (2002) and by sulfonic groups as described in PCT Publication No. WO 94/28073. The best results to date have been obtained using sulfonic substituent groups, which provide for sufficient solubility and the formation of a stable LLC phase of perylene dyes.
The standard procedure for synthesizing disulfoderivatives is as follows. Controlled amounts of PTCA DBI and oleum are added to a volume of chlorosulfonic acid. Upon termination of the reaction, the mixture is colored and diluted with water. The precipitate is filtered, washed with hydrochloric acid, and dried. This yields water-soluble dibenzimidazole perylenetetracarboxydisulfonic acid, which is dissolved in water and purified. An analysis of the system texture reveals that, beginning with a certain dye concentration, a stable hexagonal lyotropic mesophase is formed in a given temperature interval. Accordingly, a nematic phase is observed within a sufficiently narrow range of dye concentrations and temperatures. The boundaries of existence of isotropic phases, as well as two-phase transition regions, have been determined in this system.
Various dye compositions (also referred to as “inks”) used in the manufacture of polarizer films based on PTCA DBI sulfoderivatives have been patented. One example of currently available dyes has the structural formula:
where
R is H, alkyl group, halogen or alkoxy group; and Ar is substituted or unsubstituted aryl radical. This compound, described in U.S. Pat. No. 5,739,296, selectively absorbs in the spectral region of approximately 550 to 600 nm.
Another currently available dye composition is based on PTCA DBI of the formula
where R1 is H, 3(4)-CH3, 3(4)-C2H5, 3(4)-Cl, or 3(4)-Br and R2 is 4(5)-SO3H. This compound, described in SU Pat. No. 1,598,430, selectively absorbs in the region of approximately 550 to 600 nm.
LC blends of PTCA DBI sulfoderivatives with various modifying additives introduced to improve the characteristics of anisotropic films were described in PCT Publication No. WO 94/28073. One such compound has the general formula:
where
and R is H, Br, Cl, OH, Alk, or Oalk.
Indanthrone disulfoderivatives with various substituents and compositions with various organic cations have been described previously. Thin anisotropic films obtained using LLC systems based on sulfoderivatives of various organic dyes, including perylene dyes, have also been characterized with respect to their properties and structures. In particular, the properties of films obtained using perylene dye-based LLC systems were studied by I. K. Iverson, S. M. Casey, W. Seo, and S.-W. Tam-Chang in Controlling Molecular Orientation in Solid-Crystalline Phase, Langmuir 18(9), 3510 to 3516 (2002). All of the studied films were reported to possess a high degree of optical anisotropy.
The properties of thin anisotropic films obtained using LLC systems based on sulfoderivatives of organic dyes of the proposed method were reported in T. Fiske, L. Ignatov, P. Lazarev, V. Nazarov, M. Paukshto Molecular Alignment in Crystal Polarizers and Retarders, Society for Information Display, Int. Symp. Digest of Technical Papers (Boston, Mass., May 19–24, 2002), p. 566 to 569. It was established that these films possess at least partially crystalline structures. Optically anisotropic films may be formed on glass, plastic, or other substrate materials. The Violet dye used for the formation of these anisotropic films represents a blend of cis and trans isomers as described in V. Nazarov, L. Ignatov, K. Kienskaya, Electronic Spectra of Aqueous Solutions and Films Made of Liquid Crystal Ink for Thin Film Polarizers, Mol. Mater. 14(2), 153 to 163 (2001). Because they exhibit high quality optical characteristics and have dichroic ratios that approach the range of approximately 25 to 30, these films may be used as polarizers as described by Y. Bobrov, L. Blinov, L. Ignatov, G. King, V. Lazarev, Y.-D. Ma, V. Nazarov, E. Neburchilova, N. Ovchinnikova, S. Remizov, Environmental and Optical Testing of Optiva Thin Crystal Film™ Polarizers, Proceedings of the 10th SID Symposium “Advanced display technologies”, (Minsk, Republic of Belarus, Sep. 18–21, 2001), p. 23 to 30. Methods for the preparation of such films, including those with high degree of crystallinity, are described in PCT Publication No. WO 02/063,660. Thus, the aforementioned PTCA DBI sulfoderivatives are capable of forming LLC phases. Anisotropic films obtained using this LLC system possess excellent optical characteristics and exhibit good performance as polarizers.
A disadvantage of previously described water-soluble PTCA DBI sulfoderivatives is the difficulty of obtaining related anisotropic films possessing reproducible—for example from batch to batch and on different substrates in the same batch—and homogeneous—over the substrate surface—properties. Currently available film application technologies require that the process parameters, such as for example concentration, temperature, etc., be thoroughly selected and strictly followed. However, even if all the conditions of film formation are precisely followed, random local variation of the coating regime may occur. This is related to a certain probability of the formation of misorientation zones and microdefects as a result of non-uniform micro- and macrocrystallization processes in the course of solvent removal upon LLC system application onto a substrate surface. In addition, LLC systems based on currently available dyes are characterized by increased probability of non-uniform thickness of the applied coating, which also decreases reproducibility of the film parameters.
The aforementioned disadvantages complicate the formation of films possessing advantageous optical characteristics, make the technology insufficiently reproducible, and require most technological parameters to be thoroughly selected and strictly followed in each stage from application to drying. Accordingly, it is desirable to develop compounds and film application methods that avoid these pitfalls.